1. Field of the Invention
The present invention relates to a novel aluminum nitride sintered body. More specifically, the invention relates to an aluminum nitride sintered body having very excellent light transmission properties and can be particularly preferably used as a light-transmitting cover for a source of light having a high light-emitting efficiency.
2. Description of the Related Art
As light-transmitting materials, there have heretofore been used a transparent resin, a glass, a quartz and a translucent alumina which permits the transmission of visible light, depending upon the environment in which it is used and the cost. For instance, there has been used a transparent resin or a glass as a cover (window material) for a source of light having a low energy intensity and as a light-transmitting material for permitting the transmission of light of wavelengths containing relatively little ultraviolet rays. Further, a material containing quartz or alumina has been used as a light-transmitting cover for a source of light that contains relatively large amounts of ultraviolet rays or for a source of light that has a large energy intensity and becomes high in temperature when it is used. There is further used an alumina material having a high resistance against corrosion as a light-transmitting cover for a source of light that uses a corrosive gas such as a halogen gas.
In recent years, further, the sources of light have been so improved as to feature a higher light-emitting efficiency, and there has now been known, for example, a source of light containing, as a light-emitting material, a halide (and, particularly, an iodide and a bromide) of metals such as Na, Sc, Sn, Th, Tl, In, Li, Tm, Ho and Dy. However, an increase in the brightness of the source of light is accompanied by an increase in the amount of heat that generates. Therefore, the source of light having a high light-emitting efficiency is accompanied by a problem related to a material forming the light-transmitting member such as a cover adapted thereto. That is, the alumina material has a resistance against the halogen gas which, however, is not still satisfactory. Besides, the alumina material has a thermal conductivity of as small as 30 W/mK. Therefore, heat of the source of light is not radiated to a sufficient degree and it is considered that the life of the source of light is shortened.
In order to solve the above problem, Japanese Unexamined Patent Publication (Kokai) No. 2-26871 proposes an aluminum nitride having a good heat resistance, a thermal conductivity and a mechanical strength as a light-transmitting window material which permits the transmission of infrared rays and radar beams. According to the above publication, an AlN sintered body having a light transmission factor of 75% over a wavelength region of 0.2 μm to 30 μm is obtained by firing a starting material having a specified particle diameter of the starting material powder, a specified content of the metal impurities and a specified oxygen content in an inert gas atmosphere at 1700 to 2100° C.
Further, Japanese Unexamined Patent Publication (Kokai) No. 60-193254 discloses a light-emitting tube provided with a light-transmitting cover (hollow tube) made of an aluminum nitride sintered body obtained by using a starting aluminum nitride powder having a particle size distribution in which not less than 70% of the particles have sizes of 0.3D to 1.8D (D: average particle size). Examples of this publication discloses an aluminum nitride sintered body having a total light transmission factor of 84%.
According to the above known technology, it is surely possible to produce an AlN sintered body having improved light transmission property. As for the light transmission factor, however, there still remains room for improvement. That is, when the aluminum nitride sintered body is used as a light-transmitting cover for the source of light having a high light-emitting efficiency described above, it is desired that the light transmission factor in a visible range of 400 nm to 800 nm is not smaller than 85% when the reflection factor is taken into consideration. However, the transmission factor (400 nm to 800 nm) of the aluminum nitride sintered body obtained by the above known technology is 85% at the greatest.
As for the light transmission property in the ultraviolet ray region, further, the conventional light-transmitting aluminum nitride sintered body has room for improvement concerning the rising light transmission factor. The rising light transmission factor is important for realizing a high light transmission factor over a wide wavelength range of from an ultraviolet region to an infrared region in the use as a light-transmitting cover.